Toward a comprehensive understanding of solid-state core-level XPS linewidths: Experimental and theoretical studies on the Si 2p and O 1s linewidths in silicates

High resolution X-ray Photoelectron Spectroscopy (XPS) core-level Si 2p and O 1s spectra of the nonconductors alpha-SiO2 (quartz) at 120 and 300 K and vitreous SiO2 at 300 K were obtained with a Kratos Axis Ultra XPS instrument (instrumental resolution of <0.4 eV) which incorporates a unique charge compensation system that minimizes differential charge broadening on nonconductors. The Si 2p and O 1s linewidths at 300 K (similar to 1.1 and similar to 1.2 eV, respectively) are similar for all silicates (and similar to previous thin film SiO2 spectra obtained previously), showing that differential charging does not contribute significantly to our spectra. At 120 K, there is a small decrease (0.04 eV) in the Si 2p linewidth of alpha-SiO2, but no measurable decrease in O 1s linewidth. The O 1s lines are generally and distinctly asymmetric. We consider all possible sources of line broadening and show that final state vibrational broadening (FSVB) and phonon broadening are the major causes of the broad and asymmetric lines. Previous high resolution gas phase XPS studies have identified large FSVB contributions to the Si 2p spectra of SiCl4, SiF4, and Si(OCH3)(4) molecules, and this vibrational structure leads total Si 2p(3/2) linewidths of up to similar to 0.5 eV, even with individual peak linewidths of < 0.1 eV. The Si atom of Si(OCH3)(4) is an excellent analog for Si in crystalline SiO2 because the Si-O bond lengths and symmetric stretch frequencies are similar in both compounds. Similar vibrational contributions to the Si 2p and O 1s spectra of solid silicates are anticipated if the Si 2p and O 1s core-hole states produce similar changes to the Si-O bond length in both phases. To investigate the possibility, Car-Parrinello molecular dynamics calculations were performed and show that changes to Si-O bond lengths between ion and ground states (Delta r) for both Si 2p and O 1s hole states are similar for both crystalline SiO2 and gaseous SiOCH3)(4). Delta r are -0.04 angstrom for Si 2p and similar to+0.05 angstrom for O 1s in both compounds. Indeed, the vibrational envelope from the Si 2p spectrum of SiOCH3)(4), broadened to our instrumental linewidth of 0.4 eV, accounts for the majority (similar to 0.8 eV) of the Si 2p(3/2) linewidth for crystalline SiO2 (similar to 1.1 eV) with phonon broadening accounting for the remainder. The results provide excellent support for the tenet that final state vibrational splitting, as seen in the gas phase molecules, similarly affects the solid-state spectra. The calculations also indicate that the O 1s linewidths should be larger than the Si 2p linewidths, as observed in our spectra. FSVB should also lead to small peak asymmetries, as seen in the O 1s spectra. The contribution of phonon broadening to the linewidth is also evaluated and shown to be comparable to the FSVB contribution at 120 and 300 K but considerably smaller at very low temperatures.